The component elements of the neuronal circuitry, their synaptic organization, neurotransmitters and neurotransmitter receptors are studied with electron microscopy and light and electron microscopic immunohistochemistry. Monoclonal antibodies against purified benzodiazepine/GABA Cl- channel complex (BZR) have shown that BZRs are expressed on the surface of rat spinal and hippocampal neurons grown in tissue culture. In the primate retina, BZR and glutamate decarboxylase (GAD) immunoreactivities were largely non-overlapping showing that BZRs are not exclusively associated with GABAergic synapses. Thus the immunohistochemistry provides evidence for 2 major classes of GABAergic synapses, those which bind benzodiazepines and those which do not, as well as benzodiazepine binding in the absence of GABAergic synaptic transmission.